public QuerySourceBuilder <TDbContext, TDataItem> AddSortKey <TKey>(
Expression <Func <TDataItem, TKey> > exp, string?label = null)
{
if (_sortLabels != null || _sorter != null)
{
throw new InvalidOperationException(
"cannot add sort keys when sort labels and custom sorter already set");
}
if (_sortKeys == null)
{
_sortKeys = new();
}
_sortKeys.Add(SortKey <TDataItem> .For <TKey>(exp, label));
return(this);
}